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Accounts Receivable, net - Allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Movements in the allowance for doubtful accounts    
Balance at beginning of the year ¥ (2,070)  
Additions (9,504) ¥ (2,070)
Write-offs 161  
Balance at end of the year ¥ (11,413) ¥ (2,070)